(CNN) -- As a medical student who used humor to help his patients, Robin Williams' role in "Patch Adams" was touching and memorable.

But his laughter-as-medicine approach wasn't just limited to the silver screen. The kindness he brought to real-life patients, said their family members, was truly unforgettable.

"He's really the comedic Mother Teresa," Garry Kravit told CNN.

He would know.

In 2001, Kravit's nephew, David Buist, received a bleak diagnosis: hepatosplenic T-cell lymphoma. This aggressive form of cancer is so rare that, according to the National Institutes of Health, "standard treatment has yet to be established."

Buist, who was 24 years old at the time, met the devastating diagnosis with determination. "David said that he didn't want to just disappear," Kravit wrote in a CNN iReport submission, "and decided that he would help build a new Ronald McDonald house in New York before anything happened to him."

Kravit, who met Williams at a previous event, reached out asking for autographed memorabilia to help with the fund-raising effort.

Williams did that -- and much more.

Calls that made a difference

As Buist battled through the toughest months of his cancer treatment, Williams would call him.

Williams and David Buist, who was suffering from a rare form of lymphomia, pose together backstage at 2004 performance.

"You can't get a better laugh than a Robin Williams laugh," he said.

After a year of treatment, Buist was discharged from New York's Memorial Sloan-Kettering Cancer Center, and Williams invited him and his uncle to a taping of a standup comedy special in New York City in 2004.

Williams meet Buist backstage after the show where they hugged and caught up for several minutes. "He didn't care about anything but David," Kravit said.

Buist, who is 37 years old now, is in remission and married with children.

"He was a good person. He really tried to make people feel better. I am really sad he is gone," Buist said. "There is nothing like the power of laughter when you're feeling bad, and I wish that worked for him as well."

The actor's death Monday at age 63 was a stunning blow not just to his fans but for patients he brought smiles to. He was a long-time supporter of St. Jude Children's Research Hospital, appearing in multiple campaigns for the facility.

"Mr. Williams generously gave his time to raise awareness and funds for St. Jude and for our patients battling childhood cancer," the hospital said in a statement. "His humor brought bright smiles and laughter to our patients and families and his generosity deeply touched the hearts of all who knew him."